In a conventional transmission method, for example, the Synchronous Digital Hierarchy (SDH) transmission method, which is popular in optical communication and which conforms to ITU-T Recommendation G.707 (see Non Patent Literature 1), the input bit error ratio (BER) is estimated with use of a control signal that is included in overhead (OH) and called Bit Interleaved Parity (BIP). As the BIP, even parity is calculated on a bit-by-bit basis for every byte in an SDH frame. Details of the BIP are described in Non Patent Literature 2.